A method and system stores and generates anti-aliased text or lineart from compressed document image files. More specifically, a Mixed Raster Content (MRC) model represents the image as an ordered set or mask/image pairs at resolutions appropriate to the content of each layer. When using token compression, use of anti-aliased text or lineart improves text and lineart image appearance for both low and high resolution by smoothing edges and avoiding token baseline jitter.
Uncompressed grayscale or color scanned document images contain too much data for convenient on-line storage and retrieval. Lossless compression of a 300 ppi grayscale scanned image, using universal compression such as Lempel-Ziv, typically causes only a small reduction in stored data. Thus, for example, an 8 MB uncompressed image may be minimally shrunk to 4 to 7 MB after lossless compression. Compression is only minimal because most of the image data is produced as a result of scanner noise in the 3 or 4 least significant bits. Thus, some lossy compression is necessary. However, due to conflicting application requirements, there is no universal method that will fit all situations.
For example, suppose the requirement is that compression must be visually lossless. The amount of achievable compression is limited, and depends strongly on the scanning resolution. For example, at 300 ppi, simple hierarchical vector quantization (HVQ) provides a guaranteed 4× compression, with perhaps a 7× typical compression after further Lempel-Ziv coding. However, even at 8× compression, grayscale images are produced that compress to about 1.0 MB/page, which is too much for many applications.
To get a reasonable (but not lossless) image, at significantly better compression, a MRC approach may be used, in which the image is stored as ordered pairs of (mask, image) layers. Mixed Raster Content (MRC) is one approach to satisfy the compression needs of differing types of data. MRC involves separating a composite image into a plurality of masks, and separately applying an appropriate compression technique to each image mask. The document is represented by a pixel map that is decomposed into a multiple mask representation.
The masks allow the image to be painted through, and the ordering is necessary because the last pixel painted in each location is the one that is apparent to a viewer. In the most simple but non-trivial example, two (mask, image) pairs are used. The first layer is the background image, represented as a low-resolution gray or color image, and its mask is taken to cover the entire image. The second layer is the text/lineart layer, represented by a binary high-resolution, e.g., a 300 ppi or greater mask and a very low resolution foreground color image that is painted through the high resolution mask. The foreground color image can be at even lower resolution than 100 ppi.
It is possible to conform with the MRC format, and use a 300 ppi text or lineart mask, compressed lossily using connected component tokens, and a 100 ppi background image compressed with JPEG or wavelets. It is also possible to use a third (mask, image) layer pair for higher resolution embedded color images that are located by a segmentor. This third image layer may also be compressed using JPEG or wavelets. A similar approach has also been used where text or lineart is also compressed lossily using binary image tokens and wavelet compression is used on the background image.
For these MRC formats, the text or lineart mask can also be compressed losslessly using Group4, Limpel-Ziv, or arithmetic coding. However, there are several problems associated with the current use of a binary text or lineart mask. First, regardless of the compression method used on the text or lineart mask, the text or lineart, when rendered, has stair-steps on nearly horizontal or vertical lines. The text or lineart image quality suffers from severe aliasing when sub-sampled. The poor quality is also evident when viewed at a higher resolution on a cathode ray tube (CRT), where the character boundaries display the noisiness of individual pixels. Another weakness of the conventional method is the baseline jitter problem when connected component tokens are used. It is very difficult to avoid visible baseline jitter when tokens are substituted, because the vertical alignment of individual characters is susceptible to the noise on character boundaries introduced by thresholding the grayscale character image to binary.
These image quality deficits, stemming from the binary character of the text or lineart, make the conventional MRC format unacceptable for applications requiring that the compression loss is not observable, such as for a bookscanner where a visually lossless archival gray image must be saved.